This invention relates to a method of and apparatus for removal of fine particles from a gas stream, for example fly ash particulates from the gaseous emissions of a coal-fired electrical generating power station.
Along with rapid industrial growth over the last two decades, there has been an alarming increase in the discharge of harmful pollutants into the environment. Unfortunately, the necessary pollution abatement technology to minimize, or eliminate the discharge of industrial waste material and its harmful effects has not kept pace with overall technological growth. To stimulate the needed pollution control innovations, stringent standards have been imposed on industry requiring the reduction or total elimination of particulate discharge in the atmosphere.
Schwab et al U.S. Pat. Nos. 4,093,430 and 4,110,086 describe a recent technological advancement in air pollution control, in particular the removal of fine particles of 0.1 microns and 3.0 microns diameter. These patents describe a high intensity ionization system (hereafter referred to as "HII unit" or "HII stage") wherein a disc-shaped discharge electrode is inserted in the throat of a Venturi diffuser. A high D.C. voltage is imposed between the discharge electrode or cathode and the Venturi diffuser, a portion of which acts as an anode. The high voltage between the two electrodes and the particular construction of the cathode disc produces a stable corona discharge of a very high intensity. Particles in the gas which pass through the electrode gap of the Venturi diffuser are charged to very high levels in proportion to their sizes. The entrained particulated are field charged by the strong applied field and by ion impaction in the region of corona discharge between the two electrodes. The high velocity of the gas stream through the Venturi throat reduces the accumulation of space charge within the corona field established at the electrode gap and thereby improves the stability of the corona discharge between the two electrodes.
In the further HII improvement of Satterthwaite, U.S. Pat. No. 4,108,615, jets of cleaned air are introduced along the anode wall to prevent particle deposition thereon and to mechanically remove excess deposits from the anode, thus preventing the onset of back corona. Most effective operation of the HII unit requires high velocity gas flow through the throat region on the order of 75 ft/sec. The gas velocity is then reduced in the exit nozzle to a lower value of about 20 ft/sec. (average based on the exit face area of the nozzle). It is common practice to utilize an array of HII devices upstream of an otherwise standard electrostatic precipitator (hereinafter referred to as "ESP unit" or "ESP stage") as shown for example in the aforementioned Satterthwaite patent. In that illustrative arrangement, the HII stage utilizes a 3 (horizontal).times.5 (vertical) array of HII devices upstream of the ESP unit. It is apparent that such an arrangement essentially results in the introduction of multiple relatively high velocity gas jets from the HII stage. For the electrostatic precipitator to function properly, relatively low gas velocities are normally required, in the range of 5 ft/sec., and further the gas load should be evenly distributed across the inlet cross-section area of the electrostatic precipitator unit. Accordingly, most effective performance of the ESP unit requires distribution of the multiple gas jet discharges from the HII unit so that gas is uniformly directed to the ESP unit. The ESP unit comprises a series of parallel spaced plates and a multiplicity of wires equally spaced between each pair of adjacent plates and positioned at intervals in the longitudinal flow direction from the ESP inlet to a gas discharge end and oriented with the wire length normal to the direction of gas flow.
One prior art method to distribute the multiple source gas from the HII stage to the ESP stage has been to utilize a suitable plenum chamber between the two stages. The suitable chamber for such purposes must be sufficiently large to allow the high velocity jets to merge so that the gas flow essentially becomes uniform throughout the available cross-sectional area. Under those conditions, the uniformly distributed gas flows into the inlet of an ESP unit and thereby is uniformly processed by the ESP unit. Although incorporating a suitably large chamber between the HII stage and ESP stage can solve the fluid flow distribution problem, such a large chamber is detrimental from a space charge standpoint. The purpose of the HII unit is to develop high charge on the small particulate matter to be removed. Once that charge is imparted to the particles, it is highly desirable for those particles to be immediately subjected to the collector plates associated with the ESP unit. Without immediate exposure to collection plates, the space charge associated with the highly charged particles (hereinafter referred to as "charged dust cloud") has undesirable tendencies to either degrade charge level or cause collection of the dust particles on available surfaces in the intermediate plenum chamber. Also flow of such a highly charged dust cloud can cause buildup of non-uniformity within the dust cloud which is deterimental to most effective collection of the dust particles in the ESP stage.
It appears from the foregoing that the need for a large chamber for effective flow distribution and a small chamber to minimize space charge problems are conflicting requirements for a highly efficient HII-ESP system.
An object of this invention is to provide an improved high intensity ionization-electrostatic precipitation system for separation of particles from gas streams.
Another object is to provide an improved gas flow distribution system between the high intensity ionizer and electrostatic precipitator stages of a particulate collection system.
A further object is to provide an improved HII-ESP gas flow distribution system which does not require a large intermediate chamber.
An additional object is to provide an improved HII-ESP gas flow distribution system between the two stages in which deposition of the dust particulate matter is minimized.
A still further object is to provide an improved HII-ESP gas flow distribution system between the two stages in which the fluid pressure drop in the direction of gas flow is minimized.
Other objects and advantages will be apparent from the ensuing disclosure and appended claims.